The invention relates to an electrodeless low-pressure discharge lamp comprising
a discharge vessel with a discharge space containing an ionizable vapour and rare gas, the discharge vessel having a cavity at an end portion, PA1 an evacuated outer bulb surrounding the discharge vessel, which outer bulb has a cavity which enters the cavity of the discharge vessel, PA1 a body of soft magnetic material enclosed in the cavities, which body has an end near the end portion of the discharge vessel and is supported at said end by a thermal insulator, PA1 an electric coil inside the cavity of the discharge vessel around said body of soft magnetic material, PA1 a heat-repelling envelope of electrically insulating material between the body of soft magnetic material and the discharge space.
Such a lamp is known from EP 88201 243 A1.
Lamps of the said type with, for example, sodium or a metal halide as the ionizable vapour have a relatively high optimal operating temperature. Thus the optimal lowest temperature of the discharge vessel is 260.degree. C. if sodium vapour is the ionizable vapour. This means that the lamp requires a good thermal insulation of the discharge vessel. Thermal losses of the lamp may be reduced by restricting the flow of heat through the body of soft magnetic material.
Soft magnetic materials of low retentivity, however, have a low heat resistance. The specific magnetic losses increase with increasing temperature, while at an increased temperature, moreover, the magnetic permeability of said materials starts to decrease.
In proportion as the body of soft magnetic material is thermally better insulated from the discharge space, the heat insulation at the end of the body may have a higher thermal resistance without a critical temperature in said body being exceeded. A higher thermal resistance of the heat insulation leads to a better luminous efficacy of the lamp.
Materials which are suitable for use as a heat-repelling envelope, such as aerogels of Al.sub.2 O.sub.3 or SiO.sub.2, are expensive, as are synthetic materials having a very high thermal resistance. Such a heat-repelling envelope has a considerable influence on the cost price of the lamp.